gnuplot documentation

70 SET-SHOW gnuplot 4.3 119
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Let us first describe how a map/surface is drawn. The input data come from an evaluated function or
from an splot data file. Each surface consists of a sequence of separate scans (isolines). The pm3d
algorithm fills the region between two neighbouring points in one scan with another two points in the
next scan by a gray (or color) according to z-values (or according to an additional ’color’ column, see
help for using (p. 74)) of these 4 corners; by default the 4 corner values are averaged, but this can
be changed by the option corners2color. In order to get a reasonable surface, the neighbouring scans
should not cross and the number of points in the neighbouring scans should not differ too much; of
course, the best plot is with scans having same number of points. There are no other requirements (e.g.
the data need not be gridded). Another advantage is that the pm3d algorithm does not draw anything
outside of the input (measured or calculated) region.
Surface coloring works with the following input data:
1. splot of function or of data file with one or three data columns: The gray/color scale is obtained
by mapping the averaged (or corners2color) z-coordinate of the four corners of the above-specified
quadrangle into the range [min color z,max color z] of zrange or cbrange providing a gray value in the
range [0:1]. This value can be used directly as the gray for gray maps. The normalized gray value can
be further mapped into a color — see set palette (p. 122) for the complete description.
2. splot of data file with two or four data columns: The gray/color value is obtained by using the
last-column coordinate instead of the z-value, thus allowing the color and the z-coordinate be mutually
independent. This can be used for 4d data drawing.
Other notes:
1. The term ’scan’ referenced above is used more among physicists than the term ’iso curve’ referenced
in gnuplot documentation and sources. You measure maps recorded one scan after another scan, that’s
why.
2. The ’gray’ or ’color’ scale is a linear mapping of a continuous variable onto a smoothly varying palette
of colors. The mapping is shown in a rectangle next to the main plot. This documentation refers to this
as a "colorbox", and refers to the indexing variable as lying on the colorbox axis. See set colorbox
(p. 93), set cbrange (p. 149).
3. To use pm3d coloring to generate a two-dimensional plot rather than a 3D surface, use set view
map or set pm3d map.
Syntax (the options can be given in any order):
set pm3d {
}
show pm3d
{ at }
{ interpolate , }
{ scansautomatic | scansforward | scansbackward | depthorder }
{ flush { begin | center | end } }
{ ftriangles | noftriangles }
{ clip1in | clip4in }
{ corners2color { mean|geomean|median|min|max|c1|c2|c3|c4 } }
{ hidden3d | nohidden3d }
{ implicit | explicit }
{ map }